We have thought about implementing this in the DAC8 PRO, but it would require relays with their shortcomings (since I am not aware of any semiconductor-based analog switch with low-enough THD).
The easiest way is just to simply connect the required number of channels together externally (DAC8 PRO outputs don't mind that). Each doubling of the number of summed channels will cause sqrt(2)-times decrease in RMS noise value due to its uncorrelated nature and normal distribution.
Because the full-scale output voltage remains the same, this results in 3dB dynamic range improvement for 2 summed channels, 6dB for 4 channels and 9dB for 8 channels (mono operation), pushing the dynamic range from the original 125dB to 128dB, 131dB or 134dB respectively (or reducing the RMS noise value from the original 2.4uV RMS to 1.7uV, 1.2uV or 0.84uV). Channel routing needs to be set appropriately in the menu.
The DAC8 front panel is 50mm in height, so 1U mounting is not possible unfortunately. The provided mounting brackets are 2U, but if there is enough interest, we could make a 1U variant for racks with sliding mounting points where equipment can be mounted offset from each other.
Pavel, Okto Research